Dc-Dc converter with reduced input current ripples

ABSTRACT

An improved DC-DC converter with reduced input current ripples. The converter includes a transformer, a switch, and a capacitor. The switch turns on and off alternately to store the energy in the transformer. The capacitor is connected in series with a rectifier and a secondary winding of the transformer across an input DC voltage so as to be charged by the energy released from the secondary winding through the DC power source. Accordingly, the circuit sees an input current which continues flowing through the DC power source even the switching element is turned off.

TECHNICAL FIELD

The present invention relates to a DC-DC converter, and moreparticularly to the DC-DC converter capable of reducing input currentripples for improving circuit efficiency.

BACKGROUND ART

U.S. Pat. No. 5,910,712 discloses a DC-DC converter of the type known asa fly-back converter which includes a transformer with a primary windingconnected in series with a switching element across an input DC powersource and a secondary winding connected across a smoothing outputcapacitor that is responsible for supplying an output DC voltage to aload. In operation, the switching element is controlled to turn on andoff for repetitively interrupting the input DC voltage supplied to theprimary winding so as to accumulate the energy in the primary windingwhen the switching element is on and release the corresponding energyfrom the secondary winding to charge the smoothing output capacitor whenthe switching element is off, thereby providing a smoothed DC outputvoltage to the load. Thus, it is possible to set the output DC voltageat a desired level even lower than the input DC voltage by selecting aduty cycle of the switching element.

This circuit, however, permits no input current being supplied from theinput DC power source while the switching element is off, therebysuffering from increased input current ripples. The increased ripplesresults in lowering the circuit efficiency as well as correspondingincreased input current peak which necessitates a large capacity for thetransformer with attendant increase in the bulk of the transformer.Also, since the transformer in this circuit is alone responsible forconveying the energy from the input DC power source to the load, thetransformer has to include a relatively large core in order to preventmagnetic flux saturation and is therefore made into a large bulk. Thus,it is difficult to use the transformer of compact design and to assemblethe whole circuit into a compact sufficient to be installed within alimited space.

DISCLOSURE OF THE INVENTION

In view of the above insufficiency, the present invention has beenaccomplished to provide an improved DC-DC converter with reduced inputcurrent ripples which is capable of utilizing a compact transformer forreducing the physical dimensions of the converter, as well as to providea ballast for a discharge lamp making the use of the compact DC-DCconverter. The DC-DC converter in accordance with the present inventionhas a converter input which is adapted to receive an input DC voltage,and a converter output which is adapted to be connected to a load forproving an output DC voltage to the load. The converter includes atransformer having a primary winding and a secondary winding. Theprimary winding is connected in series with a switching element acrossthe converter input. The switching element is driven to turn on and offin order to repetitively interrupt the DC input voltage and induce anenergy at the secondary winding in response to the switching elementbeing turned off. A capacitor is connected in circuit to be charged bythe energy released from the secondary winding so as to accumulate theoutput DC voltage, and is connected across the converter output toprovide the resulting output DC voltage to the load.

The characterizing feature of the present invention resides in that thecapacitor is connected in series with a rectifier and the secondarywinding so as to be charged by the energy released from the secondarywinding through the converter input. With this arrangement, the circuitsees an input current which continues flowing through the converterinput even while the switching element is turned off. Thus, nointerruption in the input DC current is assured to thereby reduce theinput current ripples and therefore the input current peak which enablesthe use of small-sized transformer for overall compact arrangement ofthe converter, yet improving the circuit efficiency.

A controller is included in the converter to determine a switchingfrequency of the switching element that is sufficiently higher than aresonance frequency given to a resonant system given by the capacitorand the secondary winding in order to restrain undesirable resonance forreliable converter operation.

In one embodiment of the present invention, the capacitor is connectedin series with the primary winding across the DC power source so as toform a closed loop of the capacitor, the secondary winding, therectifier, the converter input and the primary winding for flowing theinput current therethrough while the switching element is off. For thispurpose, the secondary winding has a polarity chosen in relation to thewinding sense of the primary winding such that the input DC voltage issuperimposed in phase upon the voltages induced at the first and secondwindings, respectively.

In another embodiment of the present invention, the capacitor isconnected in series with the secondary winding and the rectifier acrossthe DC power source in parallel with a series combination of the primarywinding and the switching element. Thus, there is established a closedloop of the capacitor, the secondary winding, the rectifier and theconverter input for flowing the input current therethrough while theswitching element is off. To this end, the secondary winding has apolarity chosen in relation to the winding sense of the primary windingsuch that the input DC voltage is superimposed in phase upon the voltageinduced at the secondary winding. Alternatively, the secondary windingmay have a polarity chosen in relation to the winding sense of theprimary winding such that the input DC voltage is superimposed inreverse phase upon the voltage induced at the secondary winding.

Preferably, a low-pass filter is connected across the converter outputin order to remove output ripples.

The DC-DC converter can be best applied to a ballast for a dischargelamp in which an inverter is connected to convert the output DC voltagefrom the converter into an AC voltage for operating the discharge lamp.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will become readily apparent with reference to thefollowing detailed description, particularly when considered inconjunction with the accompanying drawings, in which

FIG. 1 is a circuit diagram of a DC-DC converter in accordance with afirst embodiment of the present invention;

FIGS. 2A and 2B are diagram illustrating the operation of the converter;

FIG. 3 is a waveform chart explaining the operation of the converter;

FIG. 4 is a circuit diagram of a ballast for a discharge lampincorporating the above converter and an inverter;

FIG. 5 is a circuit diagram of another ballast for a discharge lampincorporating a modified converter and the like inverter;

FIG. 6 is a circuit diagram of a DC-DC converter in accordance with asecond embodiment of the present invention;

FIGS. 7A and 7B are diagram illustrating the operation of the converter;

FIG. 8 is a waveform chart explaining the operation of the converter;

FIG. 9 is a circuit diagram of a DC-DC converter in accordance with asecond embodiment of the present invention;

FIGS. 10A and 10B are diagram illustrating the operation of theconverter; and

FIG. 11 is a waveform chart explaining the operation of the converter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments will now be described with reference to theaccompanying drawings, wherein like reference numerals designatecorresponding or identical elements throughout the various drawings.

Referring now to FIG. 1, there is shown a DC-DC converter in accordancewith a first embodiment of the present invention. A converter 20 isadapted to be connected to a DC power source 10 to be supplied with aninput DC voltage therefrom and provides a regulated output DC voltage toenergize a load 60. Although the illustrated embodiment shows the DCpower source 10 which includes a battery 12 and an input filter composedof an inductor 14 and a capacitor 16, the converter can be applied tovarious DC power sources of different configurations. The converter 20includes a transformer 30 having a primary winding 31 which is connectedin series with a switching element 24 (e.g. transistor) across inputterminals 21, and a secondary winding 32 which gives antipolarity to theprimary winding and is connected in series with a rectifier, i.e., diode26 in a forward bias relation across output terminals 27. A smoothingoutput capacitor 34 is connected in series with the primary winding 31,the secondary winding 32, and the diode 26 across the input terminals 21with the capacitor 34 connected between the windings 31 and 32.Connected between the output terminals 27 of the converter 20 and theload 60 is a low-pass filter 40 composed of an inductor 41 and acapacitor 42. The switching element 24 is driven by a controller 50 toturn on and off at a high frequency of, for example, 20 kHz or more, andat a variable duty cycle based upon a detected voltage across the load60 and a detected load current I out in order to provide a constantoutput DC voltage to the load 60.

In operation, when the switching element 24 is on, an input current Iin1is drawn from the DC power source 10 to flow through the primary winding31 of the transformer 30, as indicated by an arrowed solid line in FIG.2A, so as to store the energy thereat. Upon subsequent turn off of theswitching element 24, the transformer 30 releases its energy to flow aninput current Iin2 through a closed loop of the secondary winding 32,diode 26, DC power source 10, primary winding 31, and capacitor 34, asindicated by an arrowed solid line in FIG. 2B, so as to charge thesmoothing output capacitor 34. The current Iin2 includes a current drawnfrom the DC power source 10 so as to give no interruption in the inputcurrent from the DC power source while the switching element is off,thereby reducing the input current ripples and therefore the currentpeak. When the switching element is subsequently on, the smoothingoutput capacitor 34 discharges to flow an output current Iout1 throughthe switching element 24, as indicated by an arrowed broken line in FIG.2A, for energizing the load. In this sense, the smoothing outputcapacitor 34 can be defined as an output capacitor which provides theoutput DC voltage to the load when the switching element 24 is on. Whenthe switching element is off, the load is continuously supplied with anoutput current Iout2 which is released from the secondary winding 32 toflow through the diode 26, as indicated by an arrowed broken line inFIG. 2B. As indicated by dots in the figures, the secondary winding 32has a winding sense in relation to the primary winding 31 such that theinput DC voltage from the DC voltage source is superimposed in phaseupon the voltages of the primary and secondary windings 31 and 32 whenthe switching element 24 is off.

FIG. 3 shows various currents flowing through the circuit of theconverter which demonstrates the above circuit operations. In thisfigure, an input current, which is a combination of currents Iin1 andIin2, is expressed by a current I31 flowing through the primary winding31, while the output current Iout is a combination of the currents Iout1and Iout2 flowing through the inductor 41. Current I32 denotes a currentflowing through the secondary winding 32.

As explained in the above, the input current is continuously fed fromthe DC power source 10 irrespectively of the on/off condition of theswitching element 24, the converter can successfully reduce the inputcurrent ripples and therefore the input current peak. This reduces apower requirement to the transformer 30 and therefore makes it possibleto use the transformer of a compact size. In addition, since the primarywinding 31 is connected in series with the secondary winding 32 and iscooperative therewith to charge the smoothing output capacitor 34 whenthe switching element is off, as shown in FIG. 2B, the number of turnsof the primary winding 31 is additive to that of the secondary winding32 in the function of charging the smoothing output capacitor 34. Thismeans that the secondary winding 32 can be made to have reduced numberof turns by the corresponding number of turns of the primary winding.With this result, the transformer can be further made compact ascompared to a case in which the secondary winding is alone for chargingthe capacitor 34. In addition, the reduced current ripple makes itpossible to use the capacitor 34 of less capacitance while retaining theintended function, thereby assisting to make the whole assembly compact.

FIG. 4 shows a ballast for a discharge lamp as one typical applicationof the DC-DC converter with the load being configured to include aninverter 70 providing a low frequency AC voltage of 1 kHz or less foroperating the lamp 100, and a starter 80 providing a high startingvoltage of 20 kV or more to the lamp. The inverter 70 has four switchingtransistors 71 to 74 arranged in the form of a full-bridge connection.The transistors are driven by a driver 76 such that a diagonally opposedpair of transistors 71 and 74 are simultaneously turned on and off in analternate relation to the other pair of transistors 72 and 73, thusconverting the output DC voltage from the converter into the AC voltagebeing applied to the lamp 100. The driver 76 is connected to receive alow frequency control signal of 1 kHz or less from the controller 50′ tomake the low frequency inverter output. The starter 80 includes atransformer with a primary winding 81 and a secondary winding 82 whichis connected in series with the lamp 100 in a path of feeding theinverter output. Connected across the primary winding 81 is a seriescombination of a capacitor 84 and a switch 85 which is responsible fordischarging the capacitor 84 so to induce the high starting voltage atthe secondary winding 82 for applying it to start the lamp.

The capacitor 84 is charged by a booster 90 which makes the use of avoltage appearing in the secondary winding 32 to provide a boosted DCvoltage sufficient for rapidly charging the capacitor 84. The booster 90is configured as a Cockcroft rectifier composed of diodes 91 to 94,capacitors 95 to 98, and a resistor 99. The booster 90 has its inputconnected across the diode 26 of the converter 20 and generates theboosted DC voltage from the voltage across the diode 26.

FIG. 5 shows another ballast which is identical to that of FIG. 4 exceptthat the converter 20 is somewhat modified such that the booster 90derives a voltage appearing across a series combination of the capacitor24 and the diode 26. In this connection, the diode 26 is connected inseries between the secondary winding 32 and the smoothing outputcapacitor 34, while the converter 20 retains the same operations asdiscussed in the above. Like parts are designated by like numerals foran easy reference purpose.

FIG. 6 shows a DC-DC converter 20A in accordance with a secondembodiment of the present invention which is similar to the firstembodiment except that the smoothing output capacitor 34A is connectedin series with the secondary winding 32A and the diode 26A across the DCpower source 10 in parallel relation to a series combination of theprimary winding 31A and the switching element 24A. Like parts aredesignated by like reference numerals with a suffix letter of “A”. Aswill be explained below, the secondary winding 32A has the same polarityas the primary winding 31A such that the input DC voltage issuperimposed in phase upon the voltage of the secondary windings 32Awhen the switching element 24A is off.

Operation of the converter is explained with reference to FIGS. 7A and7B. When the switching element 24A is on, DC power source 10 supplies aninput current Iin1 flowing through the primary winding 31A, as indicatedby an arrowed solid line in FIG. 7A, to store the energy at thetransformer 30A. Upon subsequent turn oft of the switching element 24A,the secondary winding 32A releases its energy to flow an output currentIout2 through the diode 26A to the load, as indicated by an arrowedbroken line in FIG. 7B, while allowing an input current Iin2 to continueflow from the DC power source 10 through the smoothing output capacitor34A, the secondary winding 32A and the diode 26A, as indicated by anarrowed solid line in the same figure, thereby charging the smoothingoutput capacitor 34A. Upon subsequent turn on of the switching element24A, the smoothing output capacitor 34A thus charged is made responsiblefor flowing an output current Iout1 through the primary winding 31A andthe switching element 24A to the load as indicated by a broken line inFIG. 7A. Also, the smoothing output capacitor 34A is responsible forflowing an output current Iout3 through the DC power source 10 to theload, as indicated another broken line, while the switching element 24Ais off.

FIG. 8 shows various currents flowing in the circuit of the converterfor demonstrating the above circuit operations. In this figure, an inputcurrent Iin1 is a combination of currents Iin1, Iin2, and Iout3, whilethe output current Iout is a combination of the currents Iout1, Iout2,and Iout3 supplied to the load. Currents I31 and I32 denote thoseflowing respectively through the primary and secondary windings 31A and32A. As confirmed by the waveform of FIG. 8, the converter of thisembodiment also assures no interruption in the input current forreducing the input current ripples as well as reducing the input currentpeak.

FIG. 9 shows a DC-DC converter 20B in accordance with a third embodimentof the present invention which is identical to the second embodimentexcept that the secondary winding 32B has its winding sense chosenoppositely to that of the second embodiment for flowing input and outputcurrents through the secondary winding 32B and the diode 26B in areverse direction. Like parts are designated by like reference numeralswith a suffix letter of “B”.

Operation of the converter is explained with reference to FIGS. 10A and10B. When the switching element 24B is on, DC power source 10 suppliesan input current Iin1 flowing through the primary winding 31B, asindicated by an arrowed solid line in FIG. 10A, to store the energy atthe transformer 30B. Upon subsequent turn off of the switching element24B, the secondary winding 32B releases its energy to flow a currentIout1 through the diode 26B to the load, as indicated by an arrowedbroken line in FIG. 10B. At the same time, the smoothing outputcapacitor 34B is cooperative with the DC power source 10 to flow anadditional current Iout2 to the load. Since the current Iout2 flowsthrough the DC power source 10, it can be regarded as an input currentwhich continues to flow, even in the off-period of the switching element24B, from the DC power source 10 to the converter, thereby reducing theinput current ripples and the input current peak as is made in theprevious embodiment. It is noted that during the on-period of theswitching element 24B, the DC power source 10 is also responsible forflowing a like current Iout3 through the smoothing output capacitor 34Bto the load.

FIG. 11 shows various currents flowing in the circuit of the aboveconverter 20B for demonstrating the above circuit operations. In thisfigure, an input current Iin1 is a combination of currents Iin1, Iin2,Iout2, and Iout3, while the output current Iout is a combination of thecurrents Iout1, Iout2, and Iout3 supplied to the load. Currents I31 andI32 denote those flowing respectively through the primary and secondarywindings 31B and 32B. Again as confirmed by the waveform of FIG. 11, theconverter of this embodiment also gives no interruption in the inputcurrent for reducing the input current ripples as well as reducing theinput current peak.

This application is based upon and claims the priority of JapanesePatent Application No. 2000-348758, filed in Japan on Nov. 15, 2000, theentire contents of which are expressly incorporated by reference herein.

What is claimed is:
 1. A DC-DC converter comprising: a converter inputwhich is adapted to receive an input DC voltage; a converter outputwhich is adapted to be connected to a load for providing an output DCvoltage to said load; a switching element which is connected across saidconverter input and is driven to turn on and off; a transformer having aprimary winding and a secondary winding, said primary winding beingconnected in series with said switching element across said converterinput to induce an energy at said secondary winding in response to saidswitching element being turned off, a capacitor which is connected incircuit to be charged by said energy released from said secondarywinding so as to accumulate said output DC voltage, and which isconnected across said converter output to provide said output DC voltageto said load, wherein said capacitor is connected in series with arectifier and said secondary winding across said converter input so asto be charged by the energy released from said secondary winding throughsaid converter input and said rectifier, said secondary winding beingconnected in circuit with said capacitor such that said secondarywinding, in response to said switching element being turned-off,releases its energy through said rectifier, said converter input, andsaid capacitor to charge said capacitor, and said secondary winding andsaid rectifier being connected in series across said converter outputsuch that said secondary winding, also in response to said switchingelement being turned-off, releases its energy through said rectifier toprovide said output DC voltage to said converter output.
 2. The DC-DCconverter as set forth in claim 1, further including: a controller whichdetermines a switching frequency of said switching element that issufficiently higher than a resonance frequency given to a resonantsystem given by said capacitor and said secondary winding so as torestrain the resonance.
 3. The DC-DC converter as set forth in claim 1,wherein said capacitor is connected in series with said primary windingacross said converter input.
 4. The DC-DC converter as set forth inclaim 3, wherein said secondary winding has a polarity chosen inrelation to the winding sense of the primary winding such that the inputDC voltage is superimposed in phase on the voltage induced at saidprimary and secondary windings.
 5. The DC-DC converter as set forth inclaim 1, wherein said capacitor is connected in series with saidsecondary winding and said rectifier across said converter input inparallel with a series combination of said primary winding and saidswitching element.
 6. The DC-DC converter as set forth in claim 5,wherein said secondary winding has a polarity chosen in relation to thewinding sense of the primary winding such that the input DC voltage issuperimposed in phase upon the voltage induced at the secondary winding.7. The DC-DC converter as set forth in claim 5, wherein said secondarywinding has a polarity chosen in relation to the winding sense of theprimary winding such that the input DC voltage is superimposed inreverse phase upon the voltage induced at the secondary winding.
 8. TheDC-DC converter as set forth in claim 1, wherein a filter is connectedacross said converter output so as to remove output ripples.
 9. TheDC-DC converter as set forth in claim 8, wherein said filter is alow-pass filter.
 10. A ballast for a discharge lamp, said ballast beinga combination of a DC-DC converter and an inverter, said DC-DC convertercomprising a converter input which is adapted to receive an input DCvoltage; a converter output which is adapted to be connected to a loadfor providing an output DC voltage to said load; a switching elementwhich is connected across said converter input and is driven to turn onand off; a transformer having a primary winding and a secondary winding,said primary winding being connected in series with said switchingelement across said converter input to induce an energy at saidsecondary winding in response to said switching element being turnedoff, a capacitor which is connected in circuit to be charged by saidenergy released from said secondary winding so as to accumulate saidoutput DC voltage, and which is connected across said converter outputto provide said output DC voltage to said load, wherein said capacitoris connected in series with a rectifier and said secondary windingacross said converter input so as to be charged by the energy releasedfrom said secondary winding through said converter input and saidrectifier, said inverter being connected to said converter output andconverting said output DC voltage into an AC voltage for operating thedischarge lamp, said secondary winding being connected in circuit withsaid capacitor such that said secondary winding, in response to saidswitching element being turned-off, releases its energy through saidrectifier, said converter input, and said capacitor to charge saidcapacitor, and said secondary winding and said rectifier being connectedin series across said converter output such that said secondary winding,also in response to said switching element being turned-off, releasesits energy through said rectifier to provide said output DC voltage tosaid converter output.